Degree Name

Master of Science (MS)

Semester of Degree Completion

2001

Thesis Director

Jonathan P. Blitz

Thesis Committee Member

Carol A. Deakyne

Abstract

The unmodified silica gel surface contains isolated silanols, vicinal silanols, and siloxane species. Silica gels can be thermally and/or chemically treated to obtain a desired surface. Hexamethyldisilazane (HMDS) was used to chemically modify the silica gel to obtain a surface that contains vicinal silanol groups and siloxane species. The silica gel was thermally treated at 600°C to create a surface that contains isolated silanols and siloxane species. Four types of silica gels (unmodified, 600°C, HMDS and 600°C/HMDS) were reacted with three boron compounds. The boron compounds used were boron trichloride (BCl3), triethyl borane (B(CH2CH3)3) and triethyl borate (B(OCH2CH3)3). To determine the behavior of the surface species with the boron compound used, infrared spectroscopy and elemental analysis techniques were used.

Infrared spectroscopy studies showed that BCl3 reacted with isolated silanol groups, vicinal silanols and siloxanes on the silica gel surfaces. The reactions of B(CH2CH3)3 and B(OCH2CH3)3 with the silica gel surfaces were found to have an effect on isolated silanols and siloxane species, but not on vicinal silanol groups. The spectra of B(CH2CH3)3 modified silca gels were compared to Al(CH2CH3)3 modified silica gels and very little difference was found. With both Al(CH2CH3)3 and B(CH2CH3)3, residual vicinal silanol bands were found in the spectra of the unmodified and HMDS silica gel.

Elemental analysis of the modified silica gels was used to determine the amount of boron chemisorbed on the silica gel surface. The reactivity trend of boron compounds was found to be BCl3>B(OCH2CH3)3> B(CH2CH3)3. The reaction of BCl3 with the silica gel surfaces was shown to have a reactivity trend of unmodified > 600°C > HMDS > 600°C/HMDS. The reactivity trend for the reaction of B(CH2CH3)3 and B(OCH2CH3)3 with the silica gel was as follows: unmodified > 600°C > 600°/HMDS > HMDS. The reactivity trend of B(CH2CH3)3 and B(OCH2CH3)3 differed from BCl3 due to the fact that vicinal silanol species are not reactive with these boron compounds. The elemental analysis of B(CH2CH3)3 modified silca gel was compared to Al(CH2CH3)3 silica gel and it was found that there was a larger amount of aluminum being adsorbed to the silica gel surface.

Ab initio calculations were performed to understand the difference in reactivity between the aluminum (AlCl3 and Al(CH2CH3)3) and boron (BCl3 and B(CH2CH3)3) compounds with an isolated silanol group. Molecular geometries were optimized completely at both the HF/6-31G(d,p) and MP2/6-31(d,p) levels of calculation. The optimum geometries were then used to compute the single-point energies that were used to evaluate reaction enthalpies and free energies. For the overall reactions, the changes in enthalpy and free energy are negative. Complete reaction pathways were determined for the AlCl3 and BCl3 reactions with an isolated silanol. Locating the transition states for Al(CH2CH3)3 and B(CH2CH3)3 reaction pathways has proven difficult.

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